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W. B. CHAPMAN 



continuing progressively until the sinus is resolved into capillaries 

 by about the tenth day of incubation. I tested this out on a 

 few normal chicks and found that the process begins about the 

 time mentioned, but that it is often complete by the sixth day 

 of incubation. I was much surprised to find that the sinus ter- 

 minalis in the operated chicks followed this general rule and 

 began to break up into capillaries at about the same time as the 

 normal. Figure 12 shows the sinus at forty-four hours, before 

 breaking-up has started. Figure 13 is from a chick of eighty 



Sinus terminalis. 



Fig. 12 A section of the sinus terminalis of <i chick of 44 hours incubation in 

 which the heart was destroyed by burning with the cautery in the region of the 

 heart rudiment. Operated at twentieth hour of incubation. 



hours incubation, and figure 15 is from a section of a chick one 

 hundred and five hours old where the vessel has been replaced by 

 a richly growing capillary plexus, which, judging by the periph- 

 eral sprouting, is highly vegetative. This was a somewhat ex- 

 treme case and in the same embryo portions of the sinus had not 

 yet begun to break up, although it had extended some distance 

 from the embryo after the operation. Figure 1(3 is a camera 

 lucida drawing of the sinus region of a normal chick of the same 

 age. This drawing more nearly represents the conditions found 

 in both normal and operated chicks of this age. 



In operated chicks of seven or eight days of age the sinus ter- 

 minalis had invariably broken up into capillaries, although the 



